What should one do who suffers from obsessive-compulsive disorder and constant emission of prostatic fluid, and doubts whether something exited during ablution or ritual bath from major impurity, which causes him hardship by repeatedly having to purify himself?

The cure for waswasah (whispers/obsessive thoughts) is to ignore it and pay no attention to it. So, no matter how much Satan whispers about something coming out, turn away from this whisper until you achieve absolute certainty that something has indeed come out. Do not investigate merely based on doubt. Perform ablution (wudu') once and pay no attention to doubt after finishing wudu' or during it, and similarly for ghusl (ritual bath). If you are certain that madhiy (prostatic fluid) has exited, purify it and perform wudu' from it. If you suffer from incontinence of madhiy, perform wudu' for each prayer after its time has begun, and your ablution will not be invalidated by its exit according to the Maliki school. The religion of Allah is ease, so seek Allah's help and strive against waswasah.